Cofilin-1 is involved in regulation of actin reorganization during influenza A virus assembly and budding

Influenza A virus (IAV) assembly and budding on host cell surface plasma membrane requires actin cytoskeleton reorganization. The underlying molecular mechanism involving actin reorganization remains unclarified. In this study, we found that the natural antiviral compound petagalloyl glucose (PGG) inhibits F-actin reorganization in the host cell membrane during the late stage of IAV infection, which are associated with the suppression of total cofilin-1 level and its phosphorylation. Knock-down of cofilin-1 reduces viral yields. These findings provide the first evidence that cofilin-1 plays an important role in regulating actin reorganization during IAV assembly and budding.     

Identification of Hsc70 as an influenza virus matrix protein (M1) binding factor involved in the virus life cycle

Influenza virus matrix protein 1 (M1) has been shown to play a crucial role in the virus replication, assembly and budding. We identified heat shock cognate protein 70 (Hsc70) as a M1 binding protein by immunoprecipitation and MALDI-TOF MS. The C terminal domain of M1 interacts with Hsc70. We found that Hsc70 does not correlate with the transport of M1 to the nucleus, however, it does inhibit the nuclear export of M1 and NP, thus resulting in the inhibition of viral production. This is the first demonstration that Hsc70 is directly associated with M1 and therefore is required for viral production.     

Influenza A virus-induced caspase-3 cleaves the histone deacetylase 6 in infected epithelial cells

Histone deacetylase 6 (HDAC6) is a multi-substrate cytoplasmic enzyme that regulates many important biological processes. Recently, some reports have implicated HDAC6 in viral infection. However, nothing is known about its regulation in virus-infected cells. The data presented here for the first time demonstrate the caspase-3-mediated cleavage of HDAC6 in influenza A virus (IAV)-infected cells. HDAC6 polypeptide contains the caspase-3 cleavage motif DMAD-S at the C-terminus, and is a caspase-3 substrate. The cleavage removes most of the C-terminal ubiquitin-binding zinc finger domain from HDAC6, which could be significant for HDAC6’s role in IAV-induced apoptosis in infected cells.     

How viruses access the nucleus

Many viruses depend on nuclear proteins for replication. Therefore, their viral genome must enter the nucleus of the host cell. In this review we briefly summarize the principles of nucleocytoplasmic transport, and then describe the diverse strategies used by viruses to deliver their genomes into the host nucleus. Some of the emerging mechanisms include: (1) nuclear entry during mitosis, when the nuclear envelope is disassembled, (2) viral genome release in the cytoplasm followed by entry of the genome through the nuclear pore complex (NPC), (3) capsid docking at the cytoplasmic side of the NPC, followed by genome release, (4) nuclear entry of intact capsids through the NPC, followed by genome release, and (5) nuclear entry via virus-induced disruption of the nuclear envelope. Which mechanism a particular virus uses depends on the size and structure of the virus, as well as the cellular cues used by the virus to trigger capsid disassembly and genome release. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.     

Influenza virus infection in multinucleated skeletal myofibers

We examined the progression of the WSN influenza virus infection in isolated, multinucleated rat skeletal myofibers. Contrary to mononucleated cells, the adsorbed virions showed markedly delayed entry kinetics. Viral budding occurred on the sarcolemma, but the hemagglutinin envelope glycoprotein matured inefficiently and was poorly cleaved. Compatible with this, plaque assays indicated that infective viral particles were not formed. In situ hybridization studies showed that at low-dose infection, viral RNA production was restricted to one or a few nuclei within a myofiber. Dual in situ hybridization indicated that two different viral RNAs usually co-localized in the same nucleus or nuclei, suggesting that different viral genome segments replicated in the same nucleus. Newly synthesized viral ribonucleoprotein particles (vRNPs) did not re-enter virgin nuclei. Therefore, a single infected nucleus was able to support viral protein production, and notably, these proteins could reach hundreds of micrometers from the nucleus of origin. These results suggest that after viral disassembly in the endosome, the genome segments remained glued together and entered a myonucleus as a package. Spreading of the infection into virgin nuclei either by vRNPs or newly made virions did not occur, and thus the infection was abortive.     

Affinity purification of influenza virus ribonucleoprotein complexes from the chromatin of infected cells

Like all negative-strand RNA viruses, the genome of influenza viruses is packaged in the form of viral ribonucleoprotein complexes (vRNP), in which the single-stranded genome is encapsidated by the nucleoprotein (NP), and associated with the trimeric polymerase complex consisting of the PA, PB1, and PB2 subunits. However, in contrast to most RNA viruses, influenza viruses perform viral RNA synthesis in the nuclei of infected cells. Interestingly, viral mRNA synthesis uses cellular pre-mRNAs as primers, and it has been proposed that this process takes place on chromatin. Interactions between the viral polymerase and the host RNA polymerase II, as well as between NP and host nucleosomes have also been characterized. Recently, the generation of recombinant influenza viruses encoding a One-Strep-Tag genetically fused to the C-terminus of the PB2 subunit of the viral polymerase (rWSN-PB2-Strep) has been described. These recombinant viruses allow the purification of PB2-containing complexes, including vRNPs, from infected cells. To obtain purified vRNPs, cell cultures are infected, and vRNPs are affinity purified from lysates derived from these cells. However, the lysis procedures used to date have been based on one-step detergent lysis, which, despite the presence of a general nuclease, often extract chromatin-bound material only inefficiently. Our preliminary work suggested that a large portion of nuclear vRNPs were not extracted during traditional cell lysis, and therefore could not be affinity purified. To increase this extraction efficiency, and to separate chromatin-bound from non-chromatin-bound nuclear vRNPs, we adapted a step-wise subcellular extraction protocol to influenza virus-infected cells. Briefly, this procedure first separates the nuclei from the cell and then extracts soluble nuclear proteins (here termed the "nucleoplasmic" fraction). The remaining insoluble nuclear material is then digested with Benzonase, an unspecific DNA/RNA nuclease, followed by two salt extraction steps: first using 150 mM NaCl (termed "ch150"), then 500 mM NaCl ("ch500") (Fig. 1). These salt extraction steps were chosen based on our observation that 500 mM NaCl was sufficient to solubilize over 85% of nuclear vRNPs yet still allow binding of tagged vRNPs to the affinity matrix. After subcellular fractionation of infected cells, it is possible to affinity purify PB2-tagged vRNPs from each individual fraction and analyze their protein and RNA components using Western Blot and primer extension, respectively. Recently, we utilized this method to discover that vRNP export complexes form during late points after infection on the chromatin fraction extracted with 500 mM NaCl (ch500).     

Crucial role of the influenza virus NS2 (NEP) C-terminal domain in M1 binding and nuclear export of vRNP

The influenza virus genome replicates in the host cell nucleus, and the progeny viral ribonucleoproteins (vRNPs) are exported to the cytoplasm prior to maturation. The influenza virus NS2 protein has a nuclear export signal (NES) and binds to M1. It is therefore postulated that vRNP is exported from the nucleus by binding to NS2 through M1. However, the significance of the association between NS2 and M1 for the nuclear export of vRNP is still poorly understood. We herein demonstrate that the C-terminal domain of NS2 (residues 81–100) is essential for M1 binding and the nuclear export of progeny vRNPs.

Structured summary

MINT-8057301, MINT-8057317: NS2 (uniprotkb:P03508) binds (MI:0407) to M1 (uniprotkb:P03485) by pull down (MI:0096)